As the distribution of power sources for portable electronic devices, such as mobile communication equipment for information communication, digital cameras and camcorders, has been increased in recent years, there is a sharply increasing global demand for secondary batteries as the power sources. In particular, since the portability of the portable electronic devices is largely affected by secondary batteries, there exists a strong need for high-performance secondary batteries.
The characteristics required for secondary batteries are determined by charge-discharge characteristics, cycle life characteristics, high-rate characteristics and thermal stability and the like. In view of the above-mentioned characteristics, lithium secondary batteries have drawn attention and are thus widely used at present.
In typical lithium secondary batteries, LiCoO2 is used as a positive electrode material and carbon is used as a negative electrode material. Positive electrode materials researched and developed hitherto include LiNiO2, LiCoxNi1−xO2, LiMn2O4, etc. LiCoO2 is excellent in terms of stable charge-discharge characteristics and constant discharge voltage characteristics, but disadvantageous in that cobalt (Co) is present in a relatively small amount in nature and is thus expensive, and toxic to humans. Since LiNiO2 has problems of its difficult synthesis and poor thermal stability, it has not been put to practical use yet.
On the contrary, LiMn2O4 is the most widely used positive electrode material due to relatively low-price raw materials and easy synthesis. However, a spinel type of LiMn2O4 for 4V grade secondary batteries has a theoretical discharge capacity of about 148 mAh/g, which is lower in energy density than other positive electrode materials. In addition, since the spinel type of LiMn2O4 has a three-dimensional tunnel structure, the diffusion resistance during intercalation/deintercalation of lithium ions is high, the diffusion coefficient is low compared to LiCoO2 and LiNiO2 having a two-dimensional structure (or layered crystal structure), and the cycle life characteristics are poor due to a structural change (so-called ‘Jahn-Teller distortion’).
Thus, there is a need for a composite oxide having a layered crystal structure capable of solving the above problems, and at the same time, maintaining advantages of the manganese oxide. Generally, one equivalent amount of lithium present in a composite oxide having a layered crystal structure can participate in the charging and discharging, the composite oxide has a theoretical capacity of 285 mAh/g. In the case of LiCoO2 and LiNiO2, Li ions are diffused through the two-dimensional interlayer space, resulting in a high current density. It is thus expected to attain a high output.
In order to obtain powders of layered composite oxides and spinel type composite oxides mentioned above, a solid-state reaction process and a wet process are typically used.
The solid-state reaction process refers to a process wherein carbonates or hydroxides of each constituent element are mixed and then fired, the procedure being repeated several times. The solid-state reaction process has the following drawbacks: 1) when mixing, introduction of impurities from a ball-mill is large, 2) since a non-homogeneous reaction is likely to take place, an irregular phase is formed, 3) since control over the particle size of powder is difficult, sinterability is poor, and 4) high production temperature and long production time are required.
Unlike the solid-state reaction process, the wet process is a process wherein each constituent element is controlled in the atomic range, and includes ultrasonic spray pyrolysis. According to the ultrasonic spray pyrolysis, first, a lithium salt (e.g., lithium nitrate, lithium hydroxide, etc.), cobalt nitrate and nickel nitrate are dissolved, the resulting solution is ultrasonically sprayed and pyrolyzed to obtain a composite oxide powder of the desired shape, and the powder is thermally treated to produce a final positive electrode active material.
Despite the controllability of constituent elements in the atomic range, the ultrasonic spray pyrolysis has a problem in that when a lithium salt is used, the molar ratio of lithium to other metals in the final product is out of the preferred range (Li: (Ni1/2Mn1/2, Ni1/3Co1/3Mn1/3)). In addition, since a series of steps, including solution evaporation and pyrolysis, are carried out within a short time, the thermal hysteresis is extremely low, as compared with other conventional fired materials, thus negatively affecting the crystal growth.
Owing to the above-mentioned problems, when the ultrasonic spray pyrolysis is used to produce a positive electrode active material for a lithium secondary battery, the crystal structure of the active material is destroyed according to increasing number of charge-discharge cycles of the lithium secondary battery, and the cycle life characteristics and capacity maintenance characteristics of the lithium secondary battery are steeply deteriorated.